The present invention relates, generally, to the field of electric machines, and, in particular, to a variable speed electric machine and method for generating electric energy.
Energy demands of the world today have risen above the supply of energy from naturally existing sources like fossil fuels. Conventional energy generation methods include combustion of fossil fuels to create energy that is then transformed into electric energy. Renewable forms of energy, such as solar power, and wind power, have become increasingly important to meet the ever increasing energy demands. Wind energy is utilized to generate electric energy with the help of a turbine that, in its simplest form, has a rotor with rotor blades and a generator shaft coupled to the rotor. The wind energy causes the rotor to rotate and in turn rotate the generator shaft to generate electric energy through the generator configuration. Generating energy through such means has its own set of disadvantages that include the variability of wind speed experienced by the rotor blades.
Several modern day wind turbines include a power converter that is solely responsible for balancing the variation in the input wind speed in such a way that the output provided by the wind turbine, which is supplied to an electric grid, is of constant frequency. Power converters are expensive, since they involve electronic equipment that is manufactured to precision. Hence, utilizing only power converters to balance the variation in input speed has proven to be expensive in most wind applications.
Researchers have also utilized doubly-fed induction machines as electric generators to produce electric output with constant electric frequency. For operation, doubly-fed induction machines are required to be fed some amount of excitation energy. The excitation energy is provided through a feedback loop between the electrical grid, the wind turbine, and the induction machine. Typically, the doubly-fed induction machines require about 30% of the wind turbine power to be operational.
Hence, there is a need for an electric machine that can be operational on variable speeds and produce constant frequency output electric energy and require negligible power for operations.